Question:
Is there any difference between charging the same battery each night vs. switching back and forth from one to the other?
Iow, is one method better for the 8700 than the other?

I can't see how it makes any difference in whether you alternate them or not. The issue with most rechargeable batteries these days is that they have a certain number of "charging cycles" before they begin to loose some of their ability to obtain a full charge. I have heard that 300 charge cycles seems to be the magic number for many batteries as being the point where performance begins to suffer. The moral to this story is avoid "topping off" a partially discharged battery when possible as this counts as a charging cycle.

In your case " Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, (snip)
Despite its overall advantages, lithium-ion has its drawbacks.(snip)
Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. "

Don't worry about cycles or memory. Read the site, however, it is outstanding.